This invention relates to manually set relay valves of the type used in fluid control systems such as safety systems for oil and gas wells.
Safety systems of this type act to shut-in well production lines and other flowlines in the event of unusually high or low pressure conditions. A pilot operated relay valve is typically included in the safety system to control fluid flow to and from the fluid actuator of a gate valve which opens and closes the flowline. In normal operation, the relay is held open by fluid pressure applied by pilots which sense the flowline pressure. When the pressure in the flowline is outside of the operating range of the safety system, the pilot pressure to the relay is interrupted and the relay closes. Fluid is then bled from the actuator to effect closing of the gate valve. The relay must be manually reset to the open position after the problem which caused the abnormal pressure has been corrected.
In order to assure that a resumption of pilot pressure will not reset the relay, various types of valve constructions have been proposed to achieve automatic lockout of the slide valve which forms the valve element of the relay. Manual resetting is thus necessary before the flowline can be opened following closing of the gate valve, and the problem can be identified and corrected without the possibility of it going undetected. The automatic lockout arrangements which have been proposed in the past, such as those shown in U.S. Pat. Nos. 3,877,484, 4,094,340, and 4,145,025, are less than satisfactory in a number of respects, most notably in their complexity and lack of reliability. Difficulties in proper functioning of such devices can arise in the event of seal leakage, which is not at all uncommon with O-rings and similar seal elements, particularly after the valve has been used extensively.
The relays which have been used in the past have usually been pull type valves in which a knob on the slide valve is pulled outwardly for resetting, although push type valves have also been proposed. Existing pull type relays have a pilot port in the inner end of the valve body which creates problems with respect to automatic lockout due to the application of pilot pressure directly against the piston of the slide valve, thereby tending to unseat the slide valve when pilot pressure resumes after having been interrupted. Relays have also been provided with visual indicators which display their condition, as disclosed in U.S. Pat. Nos. 4,121,615 and 4,137,942.